Power plant systems generally generate loud sounds during operation. The sound may be emitted from many different parts of the power plant. It is often necessary to attenuate, or reduce, the sound produced from power plant systems. If such attenuation is not initially done well, costly post installation mitigations solutions may be required.
Acceptable outdoor sound levels are generally specified by local noise ordinances or other government codes, which almost always use the A-weighted noise level (dBA) as their metric. In some cases, regulatory agencies may also impose project specific noise conditions on the basis of community reaction and for maintaining an appropriate acoustic environment at the project vicinity. In the absence of a local noise ordinance, county or state laws or codes or those of a similar community may be used. Even if activity noise levels do not exceed those specified by an ordinance, community acceptance is not assured. Very low ambient levels or a noise source with an often-repeated, time-varying characteristic or strong tonal content may increase the likelihood of complaints.
The exhaust stack is often a primary noise source in gas turbine power plants. Silencers in the simple cycle stack, bypass stack and/or in the heat recovery steam generator (HRSG) are typically used to mitigate noise caused by the gas turbine exhaust stream. Stack and HRSG construction can require casing thickness to be increased or cladding with perforated sheet, metal mesh, insulation, and needle-mat to be incorporated to help decrease airborne noise. For simple cycle plants, the exhaust system cost can range from $300,000 to over $1 million, depending on performance and access requirements. Should the stack or HRSG be deemed the dominant noise source during compliance testing and additional mitigation is required, retrofitting the mitigation can be extremely expensive and challenging. Additional foundations may be required if additional silencing is needed.
Current exhaust stacks may utilize a perforated internal sheet layered with a metal mesh and/or acoustic cloth to shield and contain the acoustic insulation material within a stack wall and/or silencer panel for attenuating undesirable noise. The perforated sheet, metal mesh, and acoustic cloth allow only a portion of the sound into the acoustic insulation and reflect the remainder of the sound back into the exhaust stream. This reflection limits the effectiveness of the acoustic insulation material, resulting in the need for more silencing length (L) or additional stages of silencers to achieve the desired acoustic attenuation.